I enjoyed the roguelike mechanic of obtaining the bosses special power after you defeat them. Nice job making the bosses varied with different attacks and visuals. It's also interesting that your health does not replenish after a defeating a boss. I'm assuming you'll flesh this out more so that there are ways to increase health in-between rounds perhaps.
I died to the Spinning Mauler. He had this cool laser attack, but I couldn't run after enough to avoid them. Maybe I was supposed to dash instead.
My one suggestion would be to tighten the hit boxes. When I fought Cyclonara, it seemed like I could step into edge of the cyclone's sprite without getting damaged. Of course this is more of a polish issue, but I believe it will help make the game more enjoyable. Cheers!